One technique for sending more signals down an existing fiber optic infrastructure involves the use of multi-wavelength systems. Such systems are subject to various distortions and other negative effects that degrade the optical signal passing through the system. One class of such negative effects involves chromatic dispersion, both positive and negative. Other negative effects in multi-wavelength systems result from fiber nonlinearities, including stimulated Raman scattering (SRS) cross-talk, stimulated Brillioun scattering (SBS), cross-phase modulation (XPM), and self-phase modulation (SPM).
SRS and chromatic dispersion are generally the dominant limiting effects in multi-wavelength systems. There is typically little or no interaction between SRS and chromatic dispersion. SRS is a phenomenon depending upon power output and wavelength, total power in the fiber, a number of wavelengths used in the fiber, spacing of the optical wavelengths, fiber distance, fiber dispersion, RF frequency, and state of polarization. The interplay between these various parameters may be complex.
Fiber optic transmission systems signals are affected by two types of distortions: device distortions and fiber distortions. These distortions are due to the non-linearity of the devices (such as lasers) and of the fiber used in the optical fiber transmission system. Both the fiber and the laser may introduce distortions as a function of frequency. The magnitude and phase of the laser generator distortions may depend on frequency, temperature, and current value. The magnitude and phase of the fiber distortions may depend on frequency and on the dispersion characteristics of the fiber.
Typical distortion compensators attempt to cancel these distortions by sending signals of opposite polarity so they will cancel out the non-linear effects of the system. However, conventional systems do not accommodate distortions in a four quadrants of the real and imaginary signal axis.
Conventional device distortion compensating circuits compensate for device distortions with an inline pre-distorter (FIG. 4) or auxiliary line pre-distorter (FIG. 5). Examples include those described in U.S. Pat. Nos. 5,115,440, 4,992,754, 5,132,639, 5,252,930, and 5,798,854, the entire contents of each are incorporated by reference. Prior art inline pre-distorterers may be compact circuits that are lossy and do not cover all quadrants without greater complexity or an increase in signal loss. The devices may have a high impedance (unless lossy impedance transformers are used) such that component parasitics are more difficult to handle in a 1 GHz design. The devices may exhibit good phase control due to compact design but poor phase control due to component parasitics. Prior art auxiliary path predistorterers are large expensive circuits that offer lower loss and an ability to adjust distortion phase and amplitude. However, these devices are overly complex to handle distortion phase errors at high (1 GHz) frequencies.
More advanced devices include chromatic dispersion compensator circuits which compensate for positive chromatic dispersion, such as when a standard (e.g., single mode fiber (SMF) 28) fiber has analog or quasi analog signals at 1550 nm (e.g., U.S. Pat. Nos. 6,687,432 and 6,574,389, the entire contents of each are incorporated by reference). These devices work by varying an input signal delay as a function of frequency to handle positive chromatic dispersion effects. Chromatic dispersion compensators which compensate for composite second order (CSO) and composite triple beat (CTB) or both are described in U.S. Pat. Nos. 6,574,389 and 6,687,432, the entire contents of each are incorporated by reference. These devices operate by varying an input signal delay to overcome chromatic dispersion. These devices cannot change distortion phase to handle negative chromatic dispersion distortion. These devices also cannot handle certain types of laser distortion.
In another conventional approach, a non-linear feedback loop is used to cancel out distortions in the input signal (see e.g., U.S. Pat. No. 6,593,811, the entire contents of which are incorporated herein by reference). This is a relatively non-lossy circuit which can cover multiple quadrants due to presence of both signal polarities. This device does support 1 GHz amplifier operations and provides an ability to swap predistortion phase to compensate varying laser distortion as temperature or output power is varied. This device also enables use of uncooled coaxial lasers due to ability to adjust distortion phase in multiple quadrants. However, feedback delay limits the useful bandwidth of linearization and limits the phase control of a linearizer output. This device also does not operate over 4 quadrants (simultaneously/sequentially).
In addition to distortions in the fiber optic communication system there is also crosstalk from other wavelengths in the case that more than one wavelength is carried in a single glass fiber. This crosstalk is caused by sources such as XPM and SRS [Ref. A: Journal of Lightwave Technology, Vol. 18, p. 512, 2000] and also polarization state modulation through XPM and WDM filter crosstalk. Ref A teaches a method to reduce the effects of XPM crosstalk in an externally modulated system by using 3 wavelengths in a dual output/dual receiver system. This is an undesirably complicated system that also requires dispersion compensation to achieve XPM reduction.